The present invention is directed to a power converter and, more specifically, an impedance source power converter.
Power converters are utilized in various applications and have traditionally been constructed as either voltage source converters (V-converters) or current source converters (I-converters). In a typical V-converter, a DC voltage source feeds a main converter circuit, for example, a three-phase bridge. The DC voltage source may take various forms, such as a battery, a fuel cell stack, a diode rectifier and/or a capacitor. In a typical three-phase bridge, six switches are utilized to implement the main converter circuit. FIG. 1 depicts a traditional three-phase V-converter 100, which includes six switches S1–S6, which may each include a power transistor and an anti-parallel (i.e., free wheeling) diode to provide bi-directional current flow and uni-directional voltage blocking, that are fed by a DC voltage source 102. The switches S1–S6 are controlled by a control unit (e.g., a programmed microcontroller) 106 to provide a desired output.
The V-converter has a number of conceptual and theoretical limitations. For example, an AC output voltage of a V-converter is limited below and cannot exceed a voltage level of an associated DC voltage source or the level of the DC voltage source is greater than an AC input voltage. As such, the V-converter is a buck (step-down) inverter for DC-AC power conversion and the V-converter is a boost (step-up) rectifier (i.e., boost converter) for AC-DC power conversion. In applications where overdrive is desirable, an additional DC-DC boost converter is generally required to achieve a desired voltage level. However, such a DC-DC boost converter stage increases system cost and lowers system efficiency. With reference to FIG. 1, the upper and lower devices (i.e., the switch pairs S1/S2, S3/S4 and S5/S6) of each phase leg cannot be gated on simultaneously or a shoot-through occurs, which may cause the upper and lower devices to be destroyed. In the V-converter 100 of FIG. 1, shoot-through may occur when noise, e.g., electromagnetic interference (EMI), occurs. Further, an output LC filter, which also causes additional power loss and increases control complexity, is required to provide a sinusoidal voltage at the output of the V-converter 100.
FIG. 2 depicts a traditional three-phase I-converter 200, which includes a DC current source 202 that feeds a main converter circuit 204, which is a three-phase bridge. The DC current source 202 can be a relatively large DC inductor fed by a voltage source, such as a battery, fuel cell stack, diode rectifier or thyristor converter. As with the V-converter 100 of FIG. 1, six switches S7–S12 are used to implement the three-phase bridge 204. However, the switches of an I-converter are typically different than the switches of a V-converter and may include devices such as a gate turn-off thyristor (GTO), silicon controlled rectifier (SCR) or a power transistor, e.g., an insulated gate bipolar transistor (IGBT) with a series diode which provides uni-directional current flow and bi-directional voltage blocking. The switches S7–S12 are controlled by a control unit 206 to provide a desired output.
Unfortunately, an I-converter also has a number of conceptual and theoretical limitations. For example, an AC output voltage level of an I-converter has to be greater than the level of a DC voltage source that feeds a DC inductor or the DC voltage level produced is always smaller than an AC input voltage. As such, an I-converter is a boost inverter for DC-AC power conversion and a buck rectifier (or buck converter) for an AC-DC power conversion. For applications where a wide voltage range is desirable, an additional DC-DC buck (or boost) converter is generally required. The additional power converter stage increases system cost and lowers system efficiency. In a typical I-converter, at least one of the upper devices and one of the lower devices (i.e., switches S7–S12) have to be gated and maintained on at any time. Otherwise, an open circuit of the DC inductor occurs and destruction of the devices may occur. An open-circuit, as seen by the DC inductor, may occur under various conditions, such as when electromagnetic interference (EMI) inadvertently gates off a device that is required to be maintained on. Another attribute of an I-converter is that the switches of the I-converter have to block reverse voltage and thus require a series diode to be used in combination with high speed and high performance transistors, such as IGBTs. This prevents the direct use of low cost and high performance IGBT modules and intelligent power modules (IPMs).
In addition to the above-mentioned limitations, both the V-converter and the I-converter also have a number of other attributes that are less than desirable. For example, the V-converter and the I-converter are either a boost or a buck converter and cannot be both a buck and a boost converter. That is, the output voltage range obtainable from a V-converter and an I-converter are either lower or higher than an input voltage Further, the main converter circuits of the V-converter shown in FIG. 1 and the I-converter of FIG. 2 are not interchangeable.
What is needed is a power converter that is not subject to many of the limitations of traditional voltage source converters and current source converters.